In a backlight type display apparatus (a liquid crystal display apparatus) illuminating a display panel (e.g., a liquid crystal display module) from its backside, a plane or flat light source unit (or a backlight unit) is disposed on the backside of the display panel. Moreover, a diffusing sheet, a prism sheet, a sheet for improving luminance (a reflective polarizing plate, etc) or the like is employed in order to uniformize an illuminating light (illuminating radiation) on a display panel for using it as a plane or flat light source and to enhance the luminance in the front side of a liquid crystal display apparatus. Further, in a liquid crystal display apparatus, a polarizing plate, a phase plate or the like is used as a constituent member of a liquid crystal cell. Furthermore, a liquid crystalline material, a color filter or the like is also utilized.
More specifically, for example, as a plane or flat display apparatus of which the image display area has a flat surface (a flat type display apparatus), the apparatus as illustrated in FIG. 6 has been known. The apparatus comprises a plane or flat display unit (e.g., a transmissive liquid crystal display unit) 45 and a plane light source unit adapted to illuminate the display unit from its backside. The plane or flat light source unit comprises at least one fluorescent discharge tube (cold cathode tube) 41, a reflector 42 disposed on the back side of the fluorescent discharged tube 41 for reflecting a light, a diffusing plate 43 interposed between the fluorescent discharged tube 41, a display unit 45 for diffusing the light and uniformly illuminating the display unit 45, and a prism sheet 44 laminated on the display unit side of the diffusing plate 43. The flat display unit 45, in the case of a liquid crystal display unit, comprises a first polarizing film 46a, a first glass substrate 47a, a first electrode 48a on the glass substrate, a first alignment layer 49a laminated on the electrode, a liquid crystal layer 50, a second alignment layer 49b, a second electrode 48b, a color filter 51, a second glass substrate 47b, and a second polarizing film 46b as successively built up (laminated) in that order. In such a display apparatus, the display unit can be directly illuminated from the backside by the built-in fluorescent tube (cold cathode tube) 41.
Moreover, an apparatus which comprises a backlight unit having such a light guide as illustrated in FIG. 7 as the backlight system of the flat display apparatus of FIG. 6 has been known. This backlight unit comprises a tubular light source 51 such as a fluorescent tube (cold cathode tube) disposed adjacent to a lateral side of a light guide 54, the light guide 54 for guiding a light from the tubular light source to a display panel, a diffusing plate 53 disposed on the top (emitting side or front side) of the light guide 54, and a reflector 55 disposed opposite to the display unit of the light guide. Incidentally, the thickness of the light guide 54 at the side adjacent to the tubular light source 51 is larger than that at the other side, so that the light from the tubular light source 51 is guided by the light guide 54, reflected by the reflector 55, emerged from the emerging surface (front) of the light guide 54, diffused by the diffusing plate 53, and then incident on the plane display unit (not shown) constructed (laminated) on the diffusing plate. Incidentally, in the case where a plurality of tubular light sources are disposed on a light guide to improve the luminance of the display apparatus (in the case where fluorescent tubes are used in both sides or two or more sides of the light guide), there can be usually employed a light guide in which almost whole area thereof is approximately equal in thick.
In the bottom of the light guide, white scatterers scattering a light in a wide and radial pattern are regularly arranged in dot pattern, and form a light-scattering dot.
However, in the plane light source device employing a fluorescent discharge tube, a light guide, a diffusing plate, a prism sheet (further if necessary, a protecting film for the prism sheet) as described above, costs for raw materials and for assembling are driven up due to a large number of components, and the defective fraction is increased because exogenous materials are apt to enter between components. Incidentally, it is also considered to remove the exogenous materials, but costs for assembling are further driven up. A low-cost plane light source device is, accordingly, desirable.
Recently, as a plane light source device having a simplified structure and being low in cost, there has been, proposed a plane light source device in which a wedge-shaped reflecting groove is formed at the bottom of a light guide and the reflecting light is utilized [for example, Japanese Patent Application Laid-Open No. 231315/1999 (JP-11-231315A), Japanese Patent Application Laid-Open No. 305073/2000 (JP-2000-305073A), Japanese Patent Application Laid-Open No. 348515/2000 (JP-2000-348515A, Japanese Patent Application Laid-Open No. 352719/2000 (JP-2000-352719A), Japanese Patent Application Laid-Open No. 353413/2000 (JP-2000-353413A), Japanese Patent Application Laid-Open No. 4846/2001 (JP-2001-4846A), Japanese Patent Application Laid-Open No. 21881/2001 (JP-2001-21881A), Japanese Patent Publication No. 3131034, Japanese Patent Publication No. 3120944].
The plane light source device is capable of emitting a light from the vicinity of a front of the plane light source unit. However, the light is scattered by a scattering dot of the light guide or a diffusing sheet at a large scattering angle, and the light cannot be focused with the use of a prism sheet. It is, therefore, impossible to uniformly illuminate the intra-surface of the plane light source unit and forming a uniform plane light source is difficult.
To solve such problems, a diffusing sheet is disposed on the light guide having a wedge-shaped reflecting groove. However, in the case where the diffusing sheet is disposed, the luminance is extremely deteriorated, and TCO standard (The Swedish Confederation of Professional employee) which limits change of the luminance, when the display member is observed from an oblique direction, is unsatisfied.
Japanese Patent Application Laid-Open No. 31774/2001 (JP-2001-31774A) discloses a transmissive light-scattering sheet having an islands-in-an ocean structure composed of resins different from each other in refractive index, wherein the mean particle size of the island polymer is 0.5 to 10 μm, the ratio of the ocean polymer relative to the island polymer is 70/30 to 40/60 (weight ratio), and the thickness of the sheet is 5 to 200 μm. The literature also discloses that the scattering light is diffused within the range of a scattering angle of 5 to 50° with orienting.
Further, in a conventional liquid crystal display apparatus, an ultraviolet ray (light) leaks out from a tubular light source such as a fluorescent tube, and constituent members of the plane light source unit (for example, the diffusing sheet, the prism sheet, the sheet for improving luminance (a reflective polarizing plate, etc.), the polarizing plate, the phase plate, the liquid crystalline material and the color filter) are deteriorated in long-standing use.
Japanese Patent Application Laid-Open No. 246704/1999 (JP-11-246704A) proposes protection of a liquid crystal cell with the use of a protecting film for a polarizing plate, wherein the protecting film comprises an ultraviolet absorber (ultraviolet ray absorbing agent).
Moreover, to inhibit leakage of the ultraviolet ray, there may be proposed a method in which an ultraviolet absorbable film is disposed adjacent to a fluorescent tube in a backlight unit, or a method in which an ultraviolet absorber is added to a light guide. However, in the former method, it is necessary to use a high heat-resistant film, and in the latter method, the color (hue) changes over the whole apparatus because the ultraviolet absorber slightly absorbs visible lights.
In order to inhibit leakage of an ultraviolet ray, conversion of a slight amount of an ultraviolet ray from a fluorescent tube into visible light is proposed with the use of a fluorescent material (e.g., magnesium oxide, titanium oxide) as a white scatterer formed in the bottom of the light guide. However, even in such a method, an ultraviolet ray leaks from a backlight unit. Thus, a diffusing sheet, a prism sheet, and a sheet for improving luminance (a reflective polarizing plate, etc.) are exposed to the ultraviolet ray over the long term, and as a result these sheets are tinged with yellow. In particular, in a backlight having a wedge-shaped reflecting groove formed at the bottom of a light guide, a strong ultraviolet ray leaks from the light guide because the white scatterer composed of the fluorescent material is not employed.
It is, therefore, an object of the present invention to provide a light-diffusing film (or light-scattering film) which is capable of simplifying the structure and is useful for enhancing the luminance, and a plate (or flat) light source device using the film and a liquid crystal display apparatus (particularly a reflective liquid crystal display apparatus) using the film.
It is another object of the present invention to provide a light-diffusing film which is capable of enhancing the luminance and the visibility from the inclined direction even using a light guide having a wedge-shaped reflecting groove, and a plate (or flat) light source device using the film and a liquid crystal display apparatus (particularly a reflective liquid crystal display apparatus) using the film.
It is still another object of the present invention to provide a light-diffusing film which is capable of uniformizing the luminance of the display member, inhibiting dazzle in the display surface and moire generated in correlation between a liquid crystal cell and a plane light source, and improving the display quality level, and a plate (or flat) light source device using the film and a liquid crystal display apparatus (particularly a reflective liquid crystal display apparatus) using the film.
Moreover, a further object of the present invention is to provide a light-diffusing film which is capable of stably maintaining the display quality level over the long term, and effectively protecting constituent members from an ultraviolet ray leaked from a light source, and a plate (or flat) light source device using the film and a liquid crystal display apparatus (particularly a reflective liquid crystal display apparatus) using the film.
A still further object of the present invention is to provide a light-diffusing film which is capable of effectively protecting constituent members from leaked ultraviolet ray without using a white scatterer composed of a fluorescent material, and a plate (or flat) light source device using the film and a liquid crystal display apparatus (particularly a reflective liquid crystal display apparatus) using the film.